1. Technical Field
The present invention relates to MEMS (Micro-Electro-Mechanical Systems) sensors which are formed by microfabricating a silicon layer, and more particularly, to an MEMS sensor which has a low profile and exhibits an excellent ability to hermetically seal a movable region.
2. Related Art
MEMS sensors are formed with a movable electrode portion and a fixed electrode portion by microfabricating a silicon (Si) wafer that constitutes an SOI (Silicon on Insulator) layer. The micro sensors can be used as acceleration sensors, pressure sensors, vibrating gyro-sensors, or micro relays, depending on the operation of the movable electrode portion.
In such MEMS sensors, it is necessary to hermetically seal the movable region of the movable electrode portion so that the movable electrode portion formed in a portion of the silicon wafer can be operated within a very small distance in a clean space.
According to the packaging technique disclosed in JP-A-2000-307018, a movable electrode portion and a fixed electrode portion which are formed from a silicon wafer constituting an SOI layer are disposed between upper and lower glass substrates, and the upper and lower glass substrates are bonded together by a glass frit seal on the circumferential portion of a movable region of the movable electrode portion, whereby a sealing layer is formed by the glass frit seal.
However, according to the technique disclosed in JP-A-2000-307018, since the packaging structure has the movable electrode portion enclosed between two sheets of glass substrates, the overall thickness dimension of the sensor is large.
Moreover, since the opposing distance of the upper and lower glass substrates is determined by the thickness of the glass frit seal that hermetically seals the circumference of the movable region of the movable electrode portion, it is difficult to define the opposing distance of the glass substrates with high precision. As a result, it is difficult to provide an appropriate movable clearance (margin) between the movable electrode portion and the glass substrates, necessary for the operation of the movable electrodes.
Moreover, since the glass frit seal is melted down and solidified to form a sealing layer, it is difficult to form a sealing layer with a fine pattern. Furthermore, to prevent the molten glass frit seal from flowing into the movable region of the movable electrodes, it is necessary to leave a distance between the movable region of the movable electrodes and the sealing layer of the glass frit seal. Therefore, the overall size of the package becomes too large.
Furthermore, in this kind of MEMS sensor, for miniaturization of the sensor, it is necessary to form electrode layers or lead layers on portions where the movable region of the movable electrode portion is superimposed. Therefore, it is necessary to ensure a marginal dimension between these electrode layers or lead layers and the movable electrode layer, which makes it difficult to realize a low profile of the package.